The fuel assemblies of nuclear reactors cooled by light water, especially the fuel assemblies of pressurized-water nuclear reactors, consist of bundles of rods of great length which are held by means of spacer plates transverse relative to the bundles and by means of two end sockets.
The framework of the assemblies comprises not only the spacer plates and the sockets, but also guide tubes substituted for some rods of the bundle and fastened rigidly to the spacers and the sockets.
The cooling fluid of the reactor consisting of water circulates in the longitudinal direction of the assemblies between the rods and in contact with their outer surface.
During the functioning of the reactor, some mechanical pieces of its internal structures are liable to be damaged and come loose under the effect of corrosion and wear.
In addition, during maintenance operations on the nuclear reactor during a shutdown, some particles or pieces of small size detached by machining during a repair or conversion carried out on the reactor are liable to remain in the vessel at the time of the recommissioning of the reactor, if cleaning has not been carried out perfectly. Likewise, during the periods of maintenance of the reactor, tools or tool accessories of small size can fall into some inaccessible parts of the reactor structures or be forgotten by the maintenance personnel.
At all events, the pieces, particles or tools remaining in the vessel at the time when it is recommissioned are carried along at high speed by the circulating cooling water and come to rest in those parts of the reactor which are of complex structure and which are capable of retaining them.
In particular, the pieces or particles of small size are liable to come to rest and be jammed in the space between the rods of the assemblies. This results in risks of local heating of the fuel rods attributable to the disturbances in the circulation of the cooling fluid. Increased and accelerated corrosion of the sheath of the rod is then seen locally, and this can lead to damage and to loss of sealing of this sheath.
Such a loss of sealing results in a propagation of the fission gases throughout the entire primary circuit, and therefore in high risks of contamination.
The foreign pieces or particles circulating together with the primary fluid and forming migrating bodies can also accumulate in the lower socket or upper socket or in the spacer grids for retention of the rods. This results in thermo-hydraulic disturbances in the circulation of the cooling water which can cause a considerable reduction in the coolant flow in contact with the fuel rods.
To date, there has been no known device making it possible to extract foreign pieces or particles lodged in the assemblies of a nuclear reactor and giving rise to disturbances in its functioning.